The present invention relates to a thermosensitive recording sheet, and more particularly to a thermosensitive recording sheet comprising a support material; a thermosensitive coloring layer formed on the support material, comprising a colorless or light-colored leuco dye, and an acidic material which colors the leuco dye upon application of heat thereto; and a protective layer formed on the thermosensitive coloring layer, comprising silica powder with an oil absorption of 150% or more and with a bulk density of 5 ml/g or more, which thermosensitive recording sheet is particularly improved with respect to the resistance to chemicals, high speed coloring performance and thermal-head-matching properties.
It is conventionally known that a light-colored leuco dye reacts with an organic acidic material and is colored upon melting of the leuco dye and the acidic material under application of heat hereto.
Examples of thermosensitive recording sheets in which this reaction is applied are disclosed, for instance, in Japanese Patent Publication Nos. 43-4160 and 45-14039.
Recently, those thermosensitive recording sheets have been employed in a variety of fields, for instance, for use with recorders for measurement instruments and terminal printers for computers, facsimile apparatus, automatic ticket vending apparatus, and thermosensitive copying apparatus, and as bar-code labels.
In accordance with recent remarkable improvements in the performance of the above-mentioned apparatus and the application thereof to a variety of new fields, there is a great demand for thermosensitive recording sheets which can complement those improved apparatus. More specifically, there are demanded thermosensitive recording sheets capable of yielding sharp images with high density at low energy consumption, for use with high-speed thermal pens or heads, without generating materials which adhere, for instance, in the form of particles, to the thermal pens or heads during the recording process when heat is applied to the recording sheets through the thermal pens or heads. In the case of conventional thermosensitive sheets, during the application of heat to the thermosensitive sheets by a thermal pen or head during thermal printing, materials contained in the thermosensitive coloring layer are fused and adhere, in the form of particles, to the thermal pen or head.
The particles then stick to the thermosensitive recording sheet itself and hinder the feeding thereof, or they are transferred back to the recording sheet, leaving trailing marks on the recording sheet. When the sticky particles accumulate on the thermal pen or head, image density and image sharpness tend to decrease, and images are deformed.
These conventional thermosensitive recording sheets are also slow in thermal response, not allowing rapid recording with high image density and high image sharpness.
In a thermosensitive recording sheet with a thermosensitive coloring layer comprising a leuco dye and an acidic material which colors the leuco dye upon application of heat thereto, the coloring is caused by either the leuco dye or the acidic material or both of them being fused by the thermal energy supplied by a thermal pen or head, followed by the reaction of the leuco dye and the acidic material to form a certain color.
In order to increase the thermal coloring sensitivity of the thermosensitive recording sheet, there have been proposed methods in which a thermo-fusible material is added to the thermosensitive coloring layer, which thermo-fusible material melts at a temperature lower than the melting points of either the leuco dye or the acidic material, and is capable of melting both the leuco dye and the acidic material when melted.
Examples of such thermo-fusible materials are disclosed, for instance, in the following Japanese laid-open patent applications: nitrogen-containing compounds, such as acetamide, stearamide, m-nitroaniline, and phthalic acid dinitrile in Japanese Laid-Open Patent Application No. 49-34842; acetoacetanilide in Japanese Laid-Open Patent Application No. 52-106746; and alkylated biphenyls and biphenyl alkanes in Japanese Laid-Open Patent Application No. 53-39139.
Even methods of increasing the thermal coloring sensitivities of the thermosensitive recording sheets by use of the above-mentioned compounds, however, are not adequate with recently developed high-speed thermal heads, for instance, for new facsimile apparatus with increased transmission speeds. Furthermore, in the case of high-speed thermal pens and heads, due to quick alternations of their energized and deenergized states, heat tends to accumulate around the thermal pen or head during thermal recording. As a result, the background of the thermosensitive recording sheet is also apt to be colored by the accumulated heat.
In order to prevent the coloring of the background area by the accumulated heat around the thermal pen or head, it is necessary to increase the thermal sensitivity of the thermosensitive recording sheet in such a manner that the recording sheet is colored with high contrast by a small temperature difference and at a high speed. This type of thermal sensitivity is referred to as dynamic thermal coloring sensitivity.
By use of the above-mentioned conventional thermo-fusible materials, the coloring initiation temperature of a leuco dye and an acidic material can be decreased when a heated thermal pen or head is in static contact with the thermosensitive recording sheet employing such thermo-fusible materials, thus increasing the thermal sensitivity of the thermosensitive recording sheet. In contrast to the just mentioned dynamic thermal coloring sensitivity, this type of thermal sensitivity is referred to as static thermal coloring sensitivity.
The above-mentioned conventional thermo-fusible materials can increase the static thermal coloring sensitivity, but cannot always increase the dynamic thermal coloring sensitivity. When increasing the dynamic thermal coloring sensitivity by use of those thermo-fusible materials, it is necessary to add a large amount of the thermo-fusible materials to the thermosensitive coloring layer. However, when a large amount of the thermo-fusible materials is added to the thermosensitive coloring layer, materials contained in the thermosensitive coloring layer are apt to be fused and adhere to the thermal pen or head. Further, when a large amount of the thermo-fusible materials is added to the thermosensitive coloring layer, the coloring initiation temperature of the thermosensitive coloring layer so decreases that its preservability before use becomes poor in practice, with easy occurrence of fogging in the thermosensitive coloring layer.
When increasing the dynamic thermal coloring sensitivity of a thermosensitive recording sheet by other means, it is not always advisable, from the above-mentioned point of view, to decrease the coloring initiation temperature of the thermosensitive coloring layer.
In order, then, to increase the dynamic thermal coloring sensitivity, a method of increasing the smoothness of the surface of the thermosensitive coloring layer, and a method of decreasing the content of components which do not contribute to the thermal coloring reaction, such as fillers and binder agents, in the thermosensitive coloring layer, thereby relatively increasing the contents of the coloring material, have been proposed.
The surface of the thermosensitive coloring layer can easily be made smooth by subjecting the thermosensitive recording sheet to super-calendering. However, by that super-calendering, the surface appearance of the thermosensitive recording sheet is considerably impaired, for instance, with the background of the recording sheet colored or with the surface thereof becoming unpleasantly shiny.
Further, reduction in the amounts of fillers and binders is not always advisable. For example, in order to make the background of the thermosensitive recording sheet look white in color, and to prevent materials which adhere to the thermal pen or head from coming out of the thermosensitive coloring layer during the recording process, fillers, such as calcium carbonate, clay and urea-formaldehyde resin in the form of small particles, and water-soluble binder agents for binding the coloring components and other additives and fixing them to a support material, are added to the thermosensitive coloring layer. When the contents of these fillers and binder agents are reduced, as a matter of course, the above-mentioned objects of the addition of those fillers and binder agents cannot be attained. Consequently, the method of decreasing the content of the fillers and binder agents in the thermosensitive coloring layer is not effective, as a practical matter, for increasing the dynamic thermal coloring sensitivity.
A conventional thermosensitive recording sheet with an unprotected thermosensitive coloring layer has the shortcoming that the image areas and non-image areas formed on the thermosensitive recording sheet lose their color or discolor upon contact with chemicals which are commonly used in homes, schools, offices and other places.
For instance, when the thermosensitive recording sheet is used as labels indicating prices and/or contents which are thermally printed thereon, for instance, for use in supermarkets and other stores, there are many chances of those labels coming into contact with a variety of oils and sources, salt, vinegar and other materials, whereby the printed images on the labels lose their color and become illegible in a certain period of time. Further when the thermosensitive sheets are used in homes, schools or offices, they may also come into contact with organic solvents, plasticizers, and other chemicals, such as acids and alkali materials, contained, for instance, in adhesive agents, plastic wrapping films, erasers, writing instruments, inks and adhesive tapes, and thus the printed images on the thermosensitive recording sheet will lose their color and become illegible.
Therefore, the above-mentioned unprotected thermosensitive recording sheet is not always a secure recording medium for practical use. As a matter of course, under such circumstances, there is a great demand for a thermosensitive recording sheet improved in the above discussed shortcomings of the conventional thermosensitive recording sheets.